阅读说明：本技术 温度受控脉冲射频消融 (Temperature controlled pulsed radio frequency ablation ) 是由 I.齐伯曼 A.戈瓦里 G.阿蒂亚斯 于 2019-07-17 设计创作，主要内容包括：本发明题为“温度受控脉冲射频消融”。本发明描述的实施方案包括一种系统,该系统包括射频电流(RF电流)发生器和处理器。处理器被配置成引起RF电流发生器生成多个RF电流脉冲以用于施加到受试者的组织,脉冲中的每个脉冲具有最大值大于80W的功率和小于10s的持续时间,以及小于10s的脉冲中的相继脉冲之间的间断。处理器还被配置成当脉冲中的每一个被施加到组织时,接收指示组织的测量的温度的至少一个信号,并且响应于测量的温度来控制脉冲的功率。还描述了其它实施方案。(The invention relates to temperature controlled pulsed radio frequency ablation. The described embodiments of the invention include a system comprising a radio frequency current (RF current) generator and a processor. The processor is configured to cause the RF current generator to generate a plurality of RF current pulses for application to tissue of the subject, each of the pulses having a maximum value of power greater than 80W and a duration of less than 10s, and a discontinuity between successive ones of the pulses of less than 10 s. The processor is further configured to receive at least one signal indicative of a measured temperature of the tissue as each of the pulses is applied to the tissue, and to control the power of the pulses in response to the measured temperature. Other embodiments are also described.)

1. A system, comprising:

a radio frequency current (RF current) generator;

an electrode operatively connected to the RF current generator; and

a processor configured to:

causing the RF current generator to generate a plurality of RF current pulses for application to tissue of a subject, each of the pulses having a power with a maximum of greater than 80W and a duration of less than 10s, and a discontinuity between successive pulses of less than 10s, and

receiving at least one signal indicative of a measured temperature of the tissue from a temperature measurement device operatively connected to the processor as each of the pulses is applied to the tissue, and controlling the power of the pulses in response to the measured temperature.

2. The system of claim 1, wherein the processor is configured to drive the RF current generator to apply less than seven pulses.

3. The system of claim 1, wherein the discontinuity is between 2s and 5 s.

4. The system of claim 1, wherein the duration of each pulse is between 2s and 5 s.

5. The system of claim 1, wherein the processor is configured to drive the RF current generator to apply each of the pulses such that the power of the pulses initially rises to the maximum value and then levels off at the maximum value.

6. The system of claim 1, wherein the maximum value is greater than 100W.

7. The system of claim 6, wherein the maximum value is greater than 120W.

8. The system of claim 1, wherein the maximum value is equal to a predefined target power value.

9. The system of claim 1, wherein the processor is configured to control the power of the pulse by alternately decreasing and increasing the power of the pulse.

10. The system of claim 1, wherein the processor is configured to control the power of the pulse by decreasing the power of the pulse in response to the measured temperature approaching a threshold temperature.

11. The system of claim 10, wherein the threshold temperature is between 40 ℃ and 65 ℃.

12. The system of claim 11, wherein the threshold temperature is between 40 ℃ and 55 ℃.

13. The system of claim 1, wherein no RF energy is applied to the tissue during the interruption.

14. A method, comprising:

generating a plurality of Radio Frequency (RF) current pulses with a radio frequency current (RF current) generator for application to tissue of a subject, each of the pulses having a power with a maximum value greater than 80W and a duration of less than 10s, and a discontinuity between successive pulses of less than 10 s;

applying said pulses to said tissue through electrodes in contacting relationship with said tissue and operatively connected to said RF current generator; and

receiving at least one signal indicative of a measured temperature of the tissue from a temperature sensing device as each of the pulses is applied to the tissue, and controlling the power of the pulses in response to the measured temperature.

15. The method of claim 14, wherein the discontinuity is between 2s and 5 s.

16. The method of claim 14, wherein the duration of each pulse is between 2s and 5 s.

17. The method of claim 14, wherein the tissue comprises cardiac tissue of the subject.

18. The method of claim 14, wherein applying the pulses comprises applying each of the pulses such that the power of the pulses initially rises to the maximum value and then levels off at the maximum value.

19. The method of claim 14, wherein controlling the power of the pulse comprises decreasing the power of the pulse in response to the measured temperature approaching a threshold temperature.

20. The method of claim 19, wherein the threshold temperature is between 40 ℃ and 65 ℃.

Technical Field

The present invention relates to the field of Radio Frequency (RF) ablation, such as for treating cardiac arrhythmias.

Background

Radio Frequency (RF) ablation is a treatment that kills unwanted tissue by heat. RF ablation was initially used in the eighties of the 20 th century for the treatment of cardiac arrhythmias and has been clinically used in many diseases to date and is now the treatment of choice for certain types of arrhythmias and certain cancers. During RF ablation, electrodes are typically inserted near a target region under medical imaging guidance. Tissue surrounding the electrode in the target region is then destroyed by heating via RF current.

Us patent 9,072,518 describes an ablation system and method for ablating tissue and forming lesions using high pressure pulses. A variety of different electrophysiology devices (such as catheters, surgical probes, and clamps) can be used to position one or more electrodes at a target location. The electrodes may be connected to power supply lines and, in some cases, the power to the electrodes may be controlled on an electrode-by-electrode basis. The high voltage pulse sequence provides a total amount of heating that is generally less than that observed with thermal-based radiofrequency energy ablation protocols.

International patent application publication WO/1996/010950 describes a method for treating ventricular tachycardia that includes inserting an electrode catheter into a ventricle. The ventricular wall of the heart is in contact with the ablation electrode at the site where the abnormal electrical pathway is located. The radiofrequency is delivered through the ablation electrode to the tissue for a time sufficient to identify the site of the abnormal electrical pathway, and to preheat the tissue. A short high voltage electrical pulse is then delivered to the tissue through the same electrode, thereby forming a non-conductive lesion.

U.S. patent application publication 2017/0209208 to Govari et al, the disclosure of which is incorporated herein by reference, describes a method that includes selecting a first maximum Radio Frequency (RF) power to be delivered by an electrode in the range of 70W-100W, and selecting a second maximum RF power to be delivered by the electrode in the range of 20W-60W. The method also includes selecting an allowable force on the electrode in a range of 5g-50g, selecting a maximum allowable temperature of tissue to be ablated in a range of 55C-65C, and selecting an irrigation rate for providing irrigation fluid to the electrode in a range of 8ml/min-45 ml/min. The method further comprises performing ablation of the tissue using the selected value by initially using the first power, switching to the second power after a predefined time between 3s and 6s, and terminating the ablation after a total time of ablation between 10s and 20 s.

Disclosure of Invention

According to some embodiments of the present invention, a system is provided that includes a radio frequency current (RF current) generator and a processor. The processor is configured to cause the RF current generator to generate a plurality of RF current pulses for application to tissue of the subject, each of the pulses having a maximum value of power greater than 80W and a duration of less than 10s, and a discontinuity between successive ones of the pulses of less than 10 s. The processor is further configured to receive at least one signal indicative of a measured temperature of the tissue as each of the pulses is applied to the tissue, and to control the power of the pulses in response to the measured temperature.

In some embodiments, the processor is configured to drive the RF current generator to apply less than seven pulses.

In some embodiments, the discontinuity is between 2s and 5 s.

In some embodiments, the duration of each pulse is between 2s and 5 s.

In some embodiments, the processor is configured to drive the RF current generator to apply each of the pulses such that the power of the pulses initially rises to a maximum value and then levels off at the maximum value.

In some embodiments, the maximum value is greater than 100W.

In some embodiments, the maximum value is greater than 120W.

In some embodiments, the maximum value is equal to a predefined target power value.

In some embodiments, the processor is configured to control the power of the pulses by alternately decreasing and increasing the power of the pulses.

In some embodiments, the processor is configured to control the power of the pulse by decreasing the power of the pulse in response to the measured temperature approaching the threshold temperature.

In some embodiments, the threshold temperature is between 40 ℃ and 65 ℃.

In some embodiments, the threshold temperature is between 40 ℃ and 55 ℃.

In some embodiments, no RF energy is applied to the tissue during the interruption.

There is also provided, in accordance with some embodiments of the present invention, a method, including generating a plurality of Radio Frequency (RF) current pulses for application to tissue of a subject, each of the pulses having a maximum value of power greater than 80W and a duration of less than 10s, and a discontinuity between successive ones of the pulses of less than 10 s. The method further includes receiving at least one signal indicative of a measured temperature of the tissue as each of the pulses is applied to the tissue, and controlling the power of the pulses in response to the measured temperature.

In some embodiments, the tissue comprises cardiac tissue of the subject.

The disclosure will be more fully understood from the following detailed description of embodiments of the disclosure taken in conjunction with the accompanying drawings, in which:

drawings

Fig. 1 is a schematic view of an ablation system for performing an ablation procedure according to an embodiment of the invention;

fig. 2A, 2B, 2C, and 2D schematically illustrate a distal end of a probe for use in a system according to an embodiment of the invention; and is

Fig. 3 is a schematic illustration of an application of pulsed radio frequency ablation according to some embodiments of the present invention.

Detailed Description

SUMMARY

Radiofrequency (RF) ablation in prior art systems is typically carried out at continuous power levels of about 20-50 watts, with contact forces of about 10g, and with irrigation for a duration of about one minute. Such regimens typically provide a lesion depth of about 5 mm. To obtain greater depths (such as 6-10mm), it is often necessary to increase the duration of the application of the RF current, or to increase the power level of the current. However, both of these options may be undesirable, for example, due to the possibility of steam pop forming within the tissue.

To address this challenge, U.S. patent application publication 2017/0209208 describes a range of values for contact force and irrigation rate that facilitates the application of continuous power of approximately 100 watts. During an ablation procedure, the temperature of the tissue to be ablated is carefully monitored and recorded at a high rate. If the monitored temperature exceeds a preset maximum temperature limit, the RF power supplied to the tissue is reduced. Alternatively or in addition, the impedance to the RF energy supplied to the tissue may be monitored, and if the impedance increases beyond a preset value, the RF energy supply may be interrupted. The high power of the RF current facilitates shortening the duration of the RF current to much less than one minute. In addition to this, there is hardly any risk of steam pop formation due to monitoring of tissue temperature and/or impedance.

Embodiments of the present invention also increase the efficacy and safety of ablation procedures by applying RF energy in multiple short high power pulses, typically 100W or more, rather than continuous current. The pause after each pulse allows the tissue to cool so that subsequent pulses can be applied again at high power. During each pulse, the temperature of the tissue may be monitored as described above, and the amplitude of the pulse may be adjusted in response thereto. Advantageously, this approach facilitates achieving relatively large lesion depths quickly and safely.

Description of the System

Referring initially to fig. 1, a schematic diagram of an ablation system 12 for performing an ablation procedure is shown, in accordance with an embodiment of the present invention. By way of example, the procedure is assumed to include ablation of a portion of the myocardium of a heart 16 of a human patient 18. However, it should be understood that embodiments of the present invention may be similarly applied to any ablation procedure on biological tissue.

To perform ablation, the physician 14 inserts the probe 20 into the lumen of the patient 18 such that the distal end 22 of the probe 20 enters the patient's heart 16. The distal end 22, described in more detail below with reference to fig. 2A-2D, includes one or more electrodes 24, with the one or more electrodes 24 being caused by the physician to contact respective locations of the myocardium. Probe 20 also includes a proximal end 28, proximal end 28 being connected to an operator console 48 via a suitable electrical interface, such as a port or socket.

The system 12 is controlled by a system processor 46, the system processor 46 typically being located in an operator console 48. In general, the processor 46 may be embodied as a single processor or a group of cooperatively networked or clustered processors. The processor 46 is typically a programmed digital computing device that includes a Central Processing Unit (CPU), Random Access Memory (RAM), a non-volatile secondary storage device (such as a hard disk drive or CD ROM drive), a network interface, and/or peripheral devices. As is well known in the art, program code and/or data, including software programs, is loaded into RAM for execution and processing by the CPU and results are generated for display, output, transmission or storage. For example, the program code and/or data may be downloaded to the computer in electronic form over a network, or alternatively or additionally, it may be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. Such program code and/or data, when provided to a processor, results in a machine or special purpose computer configured to perform the tasks described herein.

During the procedure, the processor 46 generally tracks the position and orientation of the distal end 22 of the probe using any suitable method known in the art. For example, the processor 46 may use a magnetic tracking method in which a magnetic transmitter external to the patient 18 generates a signal in a coil positioned in the distal end of the probe. Produced by Webster biosensing (Biosense Webster)

The system uses such a tracking method. The tracking of the distal end 22 is typically displayed on a three-dimensional representation 60 of the heart of the patient 18 on a screen 62. The progress of the ablation procedure is also typically displayed on the screen 62 as graphics 64 and/or alphanumeric data 66.

To control the relevant components of the system 12, the processor 46 may load and execute the relevant software modules stored in the memory 50. In general, the memory 50 stores a temperature module 52, a power control module 54, a force module 56, and a flush module 58, the respective functions of which are described below. (generally, any relevant processing functions described below can be said to be performed by a processor or by a module executed by a processor to perform the function.)

The console 48 includes an RF current generator 47, the RF current generator 47 being configured to generate RF currents for the ablation procedure. The console 48 also includes controls 49 used by the physician 14 to communicate with the processor 46. Console 48 may also include any other suitable hardware or software elements to facilitate communication between processor 46 and probe 20.

Reference is now made to fig. 2A-2D, which schematically illustrate the distal end 22 of the probe 20, in accordance with an embodiment of the present invention. Fig. 2A is a cross-sectional view along the length of the probe, fig. 2B is a cross-sectional view along the cut IIB-IIB marked in fig. 2A, fig. 2C is a perspective view of a portion of the distal end, and fig. 2D is a schematic cross-sectional view of a force sensor 90 incorporated into a proximal portion 92 of the distal end.

As shown in fig. 2A, the insertion tube 70 extends along the length of the probe and is connected at the terminal end of its distal end to a conductive cap electrode 24A, which conductive cap electrode 24A is used for ablation. (the conductive cap electrode 24A is also referred to herein as an "ablation electrode" or simply as a "cap") the cap electrode 24A has an approximately planar conductive surface 84 at its distal end and a substantially circular edge 86 at its proximal end. There are typically other electrodes, such as ring electrodes 24B, proximal to the ablation electrodes 24A. Typically, insertion tube 70 comprises a flexible, biocompatible polymer, while electrodes 24A and 24B comprise a biocompatible metal, such as gold or platinum, for example. The ablation electrode 24A is typically perforated by a series of irrigation holes 72. In one embodiment, there are 36 holes 72 evenly distributed on electrode 24A.

Electrical conductors 74 convey Radio Frequency (RF) electrical energy from console 48 (FIG. 1) to electrode 24A through insertion tube 70 and thereby power the electrode to ablate the myocardial tissue in contact with the electrode. Power control module 54 controls the level of RF power dissipated via electrode 24A, as described below. During an ablation procedure, irrigation fluid flowing out through the holes 72 irrigates the tissue under treatment, and the flow rate of the fluid is controlled by the irrigation module 58. Irrigation fluid is delivered to the electrode 24A by a tube (not shown) within the insertion tube 70.

The temperature sensors 78 are mounted within the conductive cap electrode 24A at locations that are axially and circumferentially arrayed around the distal tip of the probe. In the disclosed embodiments contemplated herein, the cap 24A contains six such sensors, with one set of three sensors in a distal position near the tip and another set of three sensors in a slightly more proximal position. Such a distribution is shown by way of example only, however, a greater or lesser number of sensors may be mounted in any suitable location within the top cover. The sensor 78 may comprise a thermocouple, a thermistor, or any other suitable type of miniature temperature sensor. The sensor 78 is connected by a lead (not shown) running through the length of the insertion tube 70. Thus, the temperature signal is carried to the temperature module 52 through the lead.

In the disclosed embodiment, the top cover 24A includes a relatively thick sidewall 73 that is about 0.5mm thick to provide a desired thermal isolation system between the temperature sensor 78 and the flushing fluid within the central cavity 75 of the tip. The flushing fluid exits the cavity 75 through the aperture 72. The sensor 78 is mounted on a rod 77 which is received in a longitudinal aperture 79 in the side wall 73. The rods 77 may comprise a suitable plastic material, such as polyimide, and may be held in place at their distal ends by a suitable adhesive 81, such as epoxy. U.S. patent 9,445,725, which is incorporated herein by reference, describes a catheter having a temperature sensor mounted in a configuration similar to that described above.

The above arrangement provides a series of six sensors 78, but other arrangements, and other numbers of sensors will be apparent to those skilled in the art, and all such arrangements and numbers are included within the scope of the present invention.

In the description herein, it is assumed that the distal end 22 defines a set of orthogonal xyz axes, where the axis 95 of the distal end corresponds to the z-axis in the set. For simplicity and by way of example, the y-axis is assumed to be in the plane of the paper, the xy-plane is assumed herein to correspond to the plane defined by edge 86, and the origin of the xyz-axis is assumed to be centered.

Fig. 2D is a schematic cross-sectional view of a force sensor 90 according to an embodiment of the present invention. Sensor 90 includes a spring 94, and it is assumed herein that spring 94 includes a plurality of coils 96 connecting cap 24A to proximal portion 92. Position sensor 98 is fixed to the distal side of spring 94 and is assumed herein to include one or more coils coupled to force module 56 by conductors 100.

An RF transmitter 102 (typically a coil) is secured to the proximal side of spring 94 and RF energy for transmitter 102 is provided from console 48 via conductor 104 under the control of force module 56. RF energy from the transmitter passes through the sensor 98, generating a corresponding signal in the conductor 100 of the sensor.

In operation, when a force is applied to the top cover 24A, the sensor 98 moves relative to the emitter 102, and this movement causes a change in the sensor signal. The force module 56 uses the signal changes of the sensors to provide a measure of the force on the cap 24A. The metric typically provides the magnitude and direction of the force.

A more detailed description of a sensor similar to sensor 90 is provided in U.S. patent application 2011/0130648, which is incorporated herein by reference.

Returning to FIG. 1, the temperature module 52 receives signals from six temperature sensors 78 within the header 24A and uses the signals to determine a maximum of six measured temperatures. The temperature module is typically configured to calculate the maximum temperature at a fixed rate (e.g., such as every 20ms-40 ms). In some embodiments, the maximum temperature is determined at a frequency of at least 30 Hz. The calculated maximum temperature is also referred to herein as the measured temperature, and the measured temperature is registered as the temperature of the ablated tissue. The temperature module communicates the measured temperature value to the power control module 54 so that the power control module can control the RF current in response to the measured temperature.

The power control module 54 may provide RF power to the top cover 24A in any suitable range, such as a range of 1W to 130W, 140W, or 150W. The power control module also measures the impedance of the top cover 24A, i.e., the impedance to the RF current delivered from the top cover 24A. The impedance is typically measured at a predefined rate (e.g., such as every 400ms-600 ms). If the impedance increases from the previous impedance measurement beyond a preset value (such as 7 Ω), the power control module may stop RF delivery to the cap 24A because the increase in impedance may indicate an unwanted change in the ablated tissue, such as charring or steam pops.

Typically, prior to an ablation procedure, the physician defines the RF pulse profile by selecting relevant parameters, such as the number of pulses, the maximum (or "target") power per pulse, the duration of each pulse, and the time between successive pulses. The power control module then causes the RF generator 47 to generate a plurality of RF current pulses for application to the tissue of the subject in accordance with the defined profile. Each of these pulses is supplied by a generator to the probe 20. When the probe applies the pulse, the power control module controls the power of the pulse in response to the measured temperature received from the temperature module 52, as described further below with reference to fig. 3. For example, the power control module may reduce the power delivered in response to the measured temperature approaching a threshold temperature set by the physician 14 in order to reduce the chance of undesirable effects, such as charring, coagulation on the cap 24A, and/or steam pops in the ablated tissue.

Generally, during the ablation stage, the processor 46 causes the screen 62 to display the values of the parameters selected by the physician. The processor 46 may also cause the screen 62 to display the progress of the RF delivery to the physician by methods known in the art. The display of the progression may be graphical (such as a simulation of lesion size as produced by ablation) and/or alphanumeric.

As explained above, the force module 56 measures the force on the cap 24A. In an embodiment, the allowable force for ablation is in the range of 5g-35 g. Similarly, the irrigation module 58 determines the rate at which irrigation fluid is delivered to the probe tip. In some embodiments of the invention, the rate may be set in the range of 8ml/min to 45 ml/min.

Pulsed RF ablation

As described above in summary and with reference to fig. 1, embodiments described herein provide for the safe application of short, high power RF current pulses to tissue (e.g., cardiac tissue) of a subject in order to obtain relatively deep lesions in a relatively short amount of time. In this regard, reference is now made to fig. 3, which is a schematic illustration of an application of pulsed RF ablation in accordance with some embodiments of the present invention.

Fig. 3 shows a plurality of RF current pulses 106 including a first pulse 106a, a second pulse 106b and a third pulse 106 c. Pulses 106 are applied by processor 46 to the tissue of patient 18 using radio frequency generator 47 and probe 20.

Fig. 3 also shows the temperature 108 at the interface between the tissue and the probe 20, as measured during application of the pulse 106. Temperature 108, which may be more simply described as the "temperature of the tissue," may be calculated by temperature module 52 by taking the maximum of the various measurements received from temperature sensor 78, as described above with reference to FIG. 1.

Fig. 3 also shows the power 110 of the pulse 106. The amplitude-and thus the power-of each pulse 106 is controlled by the processor 46 in response to the measured temperature 108, as described in detail below.

The pulse 106, temperature 108, and power 110 are plotted along a common time axis, with a particular time of importance t0.. Although the data in fig. 3 are based on the results of an actual experimental protocol performed, specific numerical values are omitted from fig. 3 for the sake of generalization.

Typically, each pulse 106 has a maximum power of greater than 80W (such as greater than 100W or 120W). (in other words, the power per pulse may exceed 80W, 100W or 120W at least one instant during the application of the pulse.) typically, the duration of each pulse is less than 10 s; for example, the duration of the pulse may be between 2s and 5 s. Thus, for example, the duration between time t0 (the time at which the first pulse 106a begins) and time t7 (the time at which the first pulse ends) may be between 2s and 5 s.

Each pair of successive pulses is separated by an interruption during which generally no energy is applied to the tissue. Typically, the discontinuity is less than 10s (such as between 2s and 5 s). For example, the duration between time t7 and time t8 (the time at which the second pulse begins) may be between 2s and 5 s. As described above in the summary, the discontinuities facilitate cooling of the tissue between pulse applications.

Typically, each of the pulses is applied such that the power of the pulse initially rises to the aforementioned maximum power value and then levels off at the aforementioned maximum power value. Typically, this maximum value is equal to the predefined target power value P, which, as mentioned above, may be greater than 80W, 100W or 120W. After this initial plateau, the power of the pulse typically oscillates when the pulse is controlled in response to the measured temperature of the tissue, as described in additional detail immediately below.

As each pulse is applied, the processor 46 controls the power of the pulse in response to the measured temperature 108. For example, the processor may reduce the power of the pulse in response to the measured temperature approaching, reaching, or exceeding a threshold temperature T (which may be, for example, between 40 ℃ and 65 ℃, such as between 40 ℃ and 55 ℃). Similarly, the processor may increase the power of the pulse in response to the measured temperature being substantially less than T. Thus, in general, the processor alternately decreases and increases the power of each pulse in response to the temperature 108.

In some embodiments, to control each pulse, the processor continuously uses the temperature readings to calculate a fractional change in the required power, and then adjusts the power of the pulse by that required change. For example, as described in U.S. patent application publication 2017/0209208, the desired score change may be

And

of where T is_tFor the currently measured temperature, T_t-1For a previously measured temperature, P_tIs the current pulse power and a and b are constants. (in one embodiment, for example, a-10 and b-1.)

Fig. 3 illustrates the use of the above-described control technique. In particular, the power of the first pulse is selected fromTime t0 rises until the target power is reached at time t 1. Subsequently, the power is not increased additionally, since

Is empty. At time T2, temperature 108 is sufficiently close to T that

Is negative and, thus, the power is reduced. The power continues to decrease until time t 3. At time t3, the temperature has dropped sufficiently that the calculated required power change is again positive, and so the power is again increased until the target power is again reached at time t 4. Subsequently, due to the high temperature, the power briefly levels off for a second time before decreasing again. At time t5, the power is again increased. Finally, at time t6, the processor rapidly reduces the power to zero so that the duration of the pulse does not exceed the predefined pulse duration. Similar oscillations of the power 110 can be observed for the second pulse and the third pulse.

In general, the ablation procedure may include applying any number of pulses greater than one. In general, however, less than seven pulses may be used to achieve a desired lesion depth, such that less than seven (e.g., less than six) pulses are applied. During each pulse, the power 110 may reach the target power P any number of times.

Typically, as each pulse is applied, the processor measures the impedance of the pulse, as described above with reference to fig. 1. As described further above, the processor may interrupt the application of the RF current in response to a significant increase in the measured impedance.

It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.